1. Field of the Invention
The present invention relates to an information recording medium for optically or electrically recording, erasing, rewriting and reproducing information, and a method for manufacturing the same.
2. Description of the Prior Art
Information recording media for optically recording, erasing, rewriting and reproducing information using a laser beam include phase change optical information recording media. In order to record, erase and rewrite information in a phase change optical information recording medium, a phenomenon is utilized in which a recording layer (a phase change material layer) is reversibly changed in phase between a crystalline phase and an amorphous phase. In general, when recording information, a high power laser beam (recording power) is irradiated onto a recording layer, and the portion thereof exposed to the high power laser beam is melted and cooled rapidly and changed in phase to the amorphous phase in order to record information. When erasing information, a low power laser beam (erasing power) whose power is lower than the recording power is used to irradiate a portion of the recording layer in the amorphous phase, and the recording layer is heated and cooled gradually so that the irradiated portion returns to the crystalline phase, thereby allowing recorded information to be erased. Therefore, it is possible to record or rewrite new information while erasing recorded information in the phase change optical information recording medium by irradiating the recording layer with a laser beam that is modulated in power between a high power level and a low power level (see, for example, Yoshito Tsunoda et al. “Basics and applications of optical disk storage”, The Institute of Electronics, Information and Communication Engineers, 1995, Chapter 2).
In addition, instead of irradiating the recording layer with the above-mentioned laser beam, information can be recorded onto the recording layer of an electrical phase change information recording medium by changing the state of the phase change material of the recording layer by using Joule heat generated when current is supplied thereto. This electrical phase change information recording medium utilizes Joule heat that is generated when current is supplied thereto, so as to change the state of the phase change material of the recording layer between the crystalline phase (low resistance) and the amorphous phase (high resistance). The difference in the resistances between the crystalline phase and the amorphous phase is detected in order to read information. When the current that is supplied to a recording layer thin film in the amorphous phase that is sandwiched between electrodes is increased gradually, the recording layer thin film changes its phase to the crystalline phase at a certain threshold current, and resistance drops rapidly. In addition, when a large current and short period width pulse is applied to the recording layer thin film in the crystalline phase, the recording layer thin film is melted and cooled rapidly so that it can return to the high resistance amorphous phase. Therefore, it can be used as a rewritable information recording medium. The difference in the resistance between the crystalline phase and the amorphous phase can be easily detected by standard electrical means, so this type of recording layer can be used to obtain a rewritable information recording medium (see, for example, Makoto Kikuchi “Basics of amorphous semiconductors”, Ohmsha, Ltd, 1982, Chapter 8).
Examples of phase change optical information recording media include a 4.7 GB/DVD-RAM that has been commercialized by the inventors of the present invention. The structure of the 4.7 GB/DVD-RAM is shown in FIG. 10, in which an information recording medium 12 has a seven-layer structure that includes a substrate 1, an incident side dielectric film 2, an incident side interface film 3, a recording film 4, a counterincident side interface film 5, a counterincident side dielectric film 6, a light absorption correction film 7 and a reflection film 8 arranged in this order when viewed from the laser incident side.
The incident side dielectric film 2 and the counterincident side dielectric film 6 have an optical function and a thermal function. For the optical function, an optical distance is adjusted so that the light absorption efficiency of the recording film 4 is increased and the change in reflectance between the crystalline phase and the amorphous phase is increased in order to increase signal amplitude. For the thermal function, portions such as the substrate 1 or a dummy substrate 10 that have a low resistance to heat are insulated from the recording film 4, which is heated to a high temperature during recording. A mixture of 80 mol % ZnS and 20 mol % SiO2 is conventionally used as a dielectric material, and has good transparency, a high refractive index, a low thermal conductivity, good thermal insulation properties, good mechanical characteristics and good resistance to humidity. Note that by performing calculations in accordance with the matrix method, the film thickness of the incident side dielectric film 2 and the counterincident side dielectric film 6 can be determined precisely so that conditions are created in which there will be a large difference in the quantity of reflected light between the crystalline phase and the amorphous phase of the recording film 4, and there will be a large amount of light absorption in the recording film 4 (see, for example, Hiroshi Kubota “Wave Optics”, Iwanami Shoten, 1971, Chapter 3).
The recording film 4 will not only have initial recording/rewriting capabilities, but will also have excellent archival characteristics (characteristics that allow recorded signals to be reproduced after a long period of time), and excellent archival rewrite characteristics (characteristics that allow recorded signals to be erased or rewritten after a long period of time) by employing a high-speed crystallization material that includes Ge—Sn—Sb—Te in which a portion of the Ge in a pseudo binary phase change material on the GeTe—Sb2Te3 line is substituted with Sn.
The incident side interface film 3 and the counterincident side interface film 5 function to prevent mass transfer between the incident side dielectric film 2 and the recording film 4, and between the counterincident side dielectric film 6 and the recording film 4, respectively. This mass transfer is a phenomenon in which the S in the mixture of 80 mol % ZnS and 20 mol % SiO2 used for the incident side dielectric film 2 and the counterincident side dielectric film 6 diffuses into the recording film when recording and rewriting is repeatedly performed by irradiating a laser beam onto the recording film 4. When S diffuses into the recording film, repeated rewriting capability will deteriorate (see, for example, N. Yamada et al., Japanese Journal of Applied Physics, Vol. 37 (1998), pp. 2104–2110). In order to prevent this deterioration in the repeated rewriting capability, a nitride containing Ge may be used for the incident side interface film 3 and the counterincident side interface film 5 (see, for example, Japanese Unexamined Patent Publication No. H10-275360).
The aforementioned technology has allowed an excellent rewrite capability and high reliability to be achieved, and has led to the commercialization of the 4.7 GB/DVD-RAM.
In addition, many other technologies have been studied in order to further increase the capacity of information recording media. For example, with optical information recording media, a technology for achieving high-density recording using a laser beam with a reduced spot diameter has been studied, which can be obtained by the use of a blue-violet laser beam having a wavelength shorter than that of a conventionally used red laser beam, or the use of a thinner substrate arranged on the laser beam incidence side and an objective lens having a high numerical aperture (NA). In addition, another technology has been studied in which an optical information recording medium having two information layers is used to double the storage capacity thereof, and which records and reproduces information in the two information layers by means of a laser beam that enters from one side thereof (see, for example, Japanese Unexamined Patent Publication Nos. 2000-36130 and 2002-144736).
In order to increase the capacity of an information recording medium and record with a reduced spot diameter, an optical information recording medium will be needed that can form recording marks having a good shape even if the recording mark is small. When a small spot diameter is used for recording, there will be a relatively short amount of time available to irradiate the laser beam onto the recording layer. Therefore, in order to form small recording marks, the material that forms the recording layer must crystallize quickly, or an interface layer having a high crystallization promotion effect must be arranged to be in contact with the recording layer.
In addition, in an optical information recording medium that records and reproduces information from one side of two information layers (hereinafter sometimes referred to as a two-layer optical information recording medium), in order to use a laser beam that passed through an information layer that is close to the incident side of the laser beam (hereinafter referred to as a first information layer) and record and reproduce information on an information layer that is distant from the incident side of the laser beam (hereinafter referred to as a second information layer), the transparency of the first information layer must be increased by making the recording layer extremely thin. However, when the recording layer becomes thin, the number of crystalline nuclei formed when the recording layer is crystallized will decrease, and the distance in which atoms can move will shorten. Therefore, the thinner the recording layer is, the more difficult it will be for the crystalline phase to be formed (i.e., the crystallization speed will decrease).
Furthermore, the time for crystallization will shorten if the information transmission rate is increased by shortening the information recording time of the information recording medium. Accordingly, it will be necessary to enhance the crystallization ability of the recording layer in order to achieve an information recording medium that can support a high transmission rate. Moreover, compared to recording information at a low transmission rate, when recording information at a high transmission rate, the percentage of microcrystalline nuclei formed in the amorphous phase after recording will be reduced because the recording layer will rapidly cool after heating. In other words, it will be easier to obtain a more stable amorphous phase. The amorphous phase has a tendency to change to a more stable energy state after a long period of preservation, and thus when information is recorded at a high transmission rate, it will be difficult for the recording layer to further crystallize, and archival rewrite characteristics will deteriorate.
In experiments performed by the inventors of the present invention, it was revealed that the crystallization speed (crystallization ability) of the recording layer can be improved by using a material having a composition in which Sn is substituted for a portion of the Ge in a pseudo binary on the GeTe—Sb2Te3 line or adjacent thereto as the recording layer. Here, if the amount of substituted Sn is increased, the signal amplitude will be reduced because the optical change between the crystalline phase and the amorphous phase will grow smaller. In addition, because the recorded amorphous phase becomes crystallized gradually if the quantity of Sn is increased, the archival capabilities will deteriorate, especially when information is recorded at a low transmission rate.
As noted above, it is difficult to achieve both archival rewrite characteristics at a high transmission rate and archival characteristics at a low transmission rate in one information recording medium while increasing the capacity thereof.